Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
Sub::Exporter(3)      User Contributed Perl Documentation     Sub::Exporter(3)

       Sub::Exporter - a sophisticated exporter	for custom-built routines

       version 0.987

       Sub::Exporter must be used in two places.  First, in an exporting

	 # in the exporting module:
	 package Text::Tweaker;
	 use Sub::Exporter -setup => {
	   exports => [
	     qw(squish titlecase), # always works the same way
	     reformat => \&build_reformatter, #	generator to build exported function
	     trim     => \&build_trimmer,
	     indent   => \&build_indenter,
	   collectors => [ 'defaults' ],

       Then, in	an importing module:

	 # in the importing module:
	 use Text::Tweaker
	   indent   => { margin	=> 5 },
	   reformat => { width => 79, justify => 'full', -as =>	'prettify_text'	},
	   defaults => { eol =>	'CRLF' };

       With this setup,	the importing module ends up with three	routines:
       "squish", "indent", and "prettify_text".	 The latter two	have been
       built to	the specifications of the importer -- they are not just	copies
       of the code in the exporting package.

       ACHTUNG!	 If you're not familiar	with Exporter or exporting, read
       Sub::Exporter::Tutorial first!

   Why Generators?
       The biggest benefit of Sub::Exporter over existing exporters (including
       the ubiquitous is its ability to build new coderefs	for
       export, rather than to simply export code identical to that found in
       the exporting package.

       If your module's	consumers get a	routine	that works like	this:

	 use Data::Analyze qw(analyze);
	 my $value = analyze($data, $tolerance,	$passes);

       and they	constantly pass	only one or two	different set of values	for
       the non-$data arguments,	your code can benefit from Sub::Exporter.  By
       writing a simple	generator, you can let them do this, instead:

	 use Data::Analyze
	   analyze => {	tolerance => 0.10, passes => 10, -as =>	analyze10 },
	   analyze => {	tolerance => 0.15, passes => 50, -as =>	analyze50 };

	 my $value = analyze10($data);

       The package with	the generator for that would look something like this:

	 package Data::Analyze;
	 use Sub::Exporter -setup => {
	   exports => [
	     analyze =>	\&build_analyzer,

	 sub build_analyzer {
	   my ($class, $name, $arg) = @_;

	   return sub {
	     my	$data	   = shift;
	     my	$tolerance = shift || $arg->{tolerance};
	     my	$passes	   = shift || $arg->{passes};

	     analyze($data, $tolerance,	$passes);

       Your module's user now has to do	less work to benefit from it --	and
       remember, you're	often your own user!  Investing	in customized
       subroutines is an investment in future laziness.

       This also avoids	a common form of ugliness seen in many modules:
       package-level configuration.  That is, you might	have seen something
       like the	above implemented like so:

	 use Data::Analyze qw(analyze);
	 $Data::Analyze::default_tolerance = 0.10;
	 $Data::Analyze::default_passes	   = 10;

       This might save time, until you have multiple modules using
       Data::Analyze.  Because there is	only one global	configuration, they
       step on each other's toes and your code begins to have mysterious

       Generators can also allow you to	export class methods to	be called as

	 package Data::Methodical;
	 use Sub::Exporter -setup => { exports => { some_method	=> \&_curry_class } };

	 sub _curry_class {
	   my ($class, $name) =	@_;
	   sub { $class->$name(@_); };

       Because of the way that exporters and Sub::Exporter work, any package
       that inherits from Data::Methodical can inherit its exporter and
       override	its "some_method".  If a user imports "some_method" from that
       package,	he'll receive a	subroutine that	calls the method on the
       subclass, rather	than on	Data::Methodical itself.

   Other Customizations
       Building	custom routines	with generators	isn't the only way that
       Sub::Exporters allows the importing code	to refine its use of the
       exported	routines.  They	may also be renamed to avoid naming

       Consider	the following code:

	 # this	program	determines to which circle of Hell you will be condemned
	 use Morality qw(sin virtue); #	for calculating	viciousness
	 use Math::Trig	qw(:all);     #	for dealing with circles

       The programmer has inadvertently	imported two "sin" routines.  The
       solution, in modules, would be	to import only one and
       then call the other by its fully-qualified name.	 Alternately, the
       importer	could write a routine that did so, or could mess about with

       How much	easier to write:

	 # this	program	determines to which circle of Hell you will be condemned
	 use Morality qw(virtue), sin => { -as => 'offense' };
	 use Math::Trig	-all =>	{ -prefix => 'trig_' };

       and to have at one's disposal "offense" and "trig_sin" -- not to
       mention "trig_cos" and "trig_tan".

       You can configure an exporter for your package by using Sub::Exporter
       like so:

	 package Tools;
	 use Sub::Exporter
	   -setup => { exports => [ qw(function1 function2 function3) ]	};

       This is the simplest way	to use the exporter, and is basically
       equivalent to this:

	 package Tools;
	 use base qw(Exporter);
	 our @EXPORT_OK	= qw(function1 function2 function3);

       Any basic use of	Sub::Exporter will look	like this:

	 package Tools;
	 use Sub::Exporter -setup => \%config;

       The following keys are valid in %config:

	 exports - a list of routines to provide for exporting;	each routine may be
		   followed by generator
	 groups	 - a list of groups to provide for exporting; each must	be followed by
		   either (a) a	list of	exports, possibly with arguments for each
		   export, or (b) a generator

	 collectors - a	list of	names into which values	are collected for use in
		      routine generation; each name may	be followed by a validator

       In addition to the basic	options	above, a few more advanced options may
       be passed:

	 into_level - how far up the caller stack to look for a	target (default	0)
	 into	    - an explicit target (package) into	which to export	routines

       In other	words: Sub::Exporter installs a	"import" routine which,	when
       called, exports routines	to the calling namespace.  The "into" and
       "into_level" options change where those exported	routines are

	 generator  - a	callback used to produce the code that will be installed
		      default: Sub::Exporter::default_generator

	 installer  - a	callback used to install the code produced by the generator
		      default: Sub::Exporter::default_installer

       For information on how these callbacks are used,	see the	documentation
       for "default_generator" and "default_installer".

   Export Configuration
       The "exports" list may be provided as an	array reference	or a hash
       reference.  The list is processed in such a way that the	following are

	 { exports => [	qw(foo bar baz), quux => \&quux_generator ] }

	 { exports =>
	   { foo => undef, bar => undef, baz =>	undef, quux => \&quux_generator	} }

       Generators are code that	return coderefs.  They are called with four

	 $class	- the class whose exporter has been called (the	exporting class)
	 $name	- the name of the export for which the routine is being	build
	\%arg	- the arguments	passed for this	export
	\%col	- the collections for this import

       Given the configuration in the "SYNOPSIS", the following	"use"

	 use Text::Tweaker
	   reformat => { -as =>	'make_narrow', width =>	33 },
	   defaults => { eol =>	'CR' };

       would result in the following call to &build_reformatter:

	 my $code = build_reformatter(
	   { width => 33 }, # note that	-as is not passed in
	   { defaults => { eol => 'CR' } },

       The returned coderef ($code) would then be installed as "make_narrow"
       in the calling package.

       Instead of providing a coderef in the configuration, a reference	to a
       method name may be provided.  This method will then be called on	the
       invocant	of the "import"	method.	 (In this case,	we do not pass the
       $class parameter, as it would be	redundant.)

   Group Configuration
       The "groups" list can be	passed in the same forms as "exports".	Groups
       must have values	to be meaningful, which	may either list	exports	that
       make up the group (optionally with arguments) or	may provide a way to
       build the group.

       The simpler case	is the first: a	group definition is a list of exports.
       Here's the example that could go	in exporter in the "SYNOPSIS".

	 groups	 => {
	   default    => [ qw(reformat)	],
	   shorteners => [ qw(squish trim) ],
	   email_safe => [
	     reformat => { -as => 'email_format', width	=> 72 }

       Groups are imported by specifying their name prefixed be	either a dash
       or a colon.  This line of code would import the "shorteners" group:

	 use Text::Tweaker qw(-shorteners);

       Arguments passed	to a group when	importing are merged into the groups
       options and passed to any relevant generators.  Groups can contain
       other groups, but looping group structures are ignored.

       The other possible value	for a group definition,	a coderef, allows one
       generator to build several exportable routines simultaneously.  This is
       useful when many	routines must share enclosed lexical variables.	 The
       coderef must return a hash reference.  The keys will be used as export
       names and the values are	the subs that will be exported.

       This example shows a simple use of the group generator.

	 package Data::Crypto;
	 use Sub::Exporter -setup => { groups => { cipher => \&build_cipher_group } };

	 sub build_cipher_group	{
	   my ($class, $group, $arg) = @_;
	   my ($encode,	$decode) = build_codec($arg->{secret});
	   return { cipher => $encode, decipher	=> $decode };

       The "cipher" and	"decipher" routines are	built in a group because they
       are built together by code which	encloses their secret in their

       Default Groups

       If a module that	uses Sub::Exporter is "use"d with no arguments,	it
       will try	to export the group named "default".  If that group has	not
       been specifically configured, it	will be	empty, and nothing will

       Another group is	also created if	not defined: "all".  The "all" group
       contains	all the	exports	from the exports list.

   Collector Configuration
       The "collectors"	entry in the exporter configuration gives names	which,
       when found in the import	call, have their values	collected and passed
       to every	generator.

       For example, the	"build_analyzer" generator that	we saw above could be
       rewritten as:

	sub build_analyzer {
	  my ($class, $name, $arg, $col) = @_;

	  return sub {
	    my $data	  = shift;
	    my $tolerance = shift || $arg->{tolerance} || $col->{defaults}{tolerance};
	    my $passes	  = shift || $arg->{passes}    || $col->{defaults}{passes};

	    analyze($data, $tolerance, $passes);

       That would allow	the importer to	specify	global defaults	for his

	 use Data::Analyze
	   analyze  => { tolerance => 0.10, -as	=> analyze10 },
	   analyze  => { tolerance => 0.15, passes => 50, -as => analyze50 },
	   defaults => { passes	=> 10 };

	 my $A = analyze10($data);     # equivalent to analyze($data, 0.10, 10);
	 my $C = analyze50($data);     # equivalent to analyze($data, 0.15, 50);
	 my $B = analyze($data,	0.20); # equivalent to analyze($data, 0.20, 10);

       If values are provided in the "collectors" list during exporter setup,
       they must be code references, and are used to validate the importer's
       values.	The validator is called	when the collection is found, and if
       it returns false, an exception is thrown.  We could ensure that no one
       tries to	set a global data default easily:

	 collectors => { defaults => sub { return (exists $_[0]->{data}) ? 0 : 1 } }

       Collector coderefs can also be used as hooks to perform arbitrary
       actions before anything is exported.

       When the	coderef	is called, it is passed	the value of the collection
       and a hashref containing	the following entries:

	 name	     - the name	of the collector
	 config	     - the exporter configuration (hashref)
	 import_args - the arguments passed to the exporter, sans collections (aref)
	 class	     - the package on which the	importer was called
	 into	     - the package into	which exports will be exported

       Collectors with all-caps	names (that is,	made up	of underscore or
       capital A through Z) are	reserved for special use.  The only currently
       implemented special collector is	"INIT",	whose hook (if present in the
       exporter	configuration) is always run before any	other hook.

       Arguments to the	exporter (that is, the arguments after the module name
       in a "use" statement) are parsed	as follows:

       First, the collectors gather any	collections found in the arguments.
       Any reference type may be given as the value for	a collector.  For each
       collection given	in the arguments, its validator	(if any) is called.

       Next, groups are	expanded.  If the group	is implemented by a group
       generator, the generator	is called.  There are two special arguments
       which, if given to a group, have	special	meaning:

	 -prefix - a string to prepend to any export imported from this	group
	 -suffix - a string to append to any export imported from this group

       Finally,	individual export generators are called	and all	subs,
       generated or otherwise, are installed in	the calling package.  There is
       only one	special	argument for export generators:

	 -as	 - where to install the	exported sub

       Normally, "-as" will contain an alternate name for the routine.	It
       may, however, contain a reference to a scalar.  If that is the case, a
       reference the generated routine will be placed in the scalar referenced
       by "-as".  It will not be installed into	the calling package.

   Special Exporter Arguments
       The generated exporter accept some special options, which may be	passed
       as the first argument, in a hashref.

       These options are:


       These override the same-named configuration options described in

       This routine builds and installs	an "import" routine.  It is called
       with one	argument, a hashref containing the exporter configuration.
       Using this, it builds an	exporter and installs it into the calling
       package with the	name "import."	In addition to the normal exporter
       configuration, a	few named arguments may	be passed in the hashref:

	 into	    - into what	package	should the exporter be installed
	 into_level - into what	level up the stack should the exporter be installed
	 as	    - what name	should the installed exporter be given

       By default the exporter is installed with the name "import" into	the
       immediate caller	of "setup_exporter".  In other words, if your package
       calls "setup_exporter" without providing	any of the three above
       arguments, it will have an "import" routine installed.

       Providing both "into" and "into_level" will cause an exception to be

       The exporter is built by	"build_exporter".

       Given a standard	exporter configuration,	this routine builds and
       returns an exporter -- that is, a subroutine that can be	installed as a
       class method to perform exporting on request.

       Usually,	this method is called by "setup_exporter", which then installs
       the exporter as a package's import routine.

       This is Sub::Exporter's default generator.  It takes bits of
       configuration that have been gathered during the	import and turns them
       into a coderef that can be installed.

	 my $code = default_generator(\%arg);

       Passed arguments	are:

	 class - the class on which the	import method was called
	 name  - the name of the export	being generated
	 arg   - the arguments to the generator
	 col   - the collections

	 generator - the generator to be used to build the export (code	or scalar ref)

       This is Sub::Exporter's default installer.  It does what	Sub::Exporter
       promises: it installs code into the target package.

	 default_installer(\%arg, \@to_export);

       Passed arguments	are:

	 into -	the package into which exports should be delivered

       @to_export is a list of name/value pairs.  The default exporter assigns
       code (the values) to named slots	(the names) in the given package.  If
       the name	is a scalar reference, the scalar reference is made to point
       to the code reference instead.

       Sub::Exporter also offers its own exports: the "setup_exporter" and
       "build_exporter"	routines described above.  It also provides a special
       "setup" collector, which	will set up an exporter	using the parameters
       passed to it.

       Note that the "setup" collector (seen in	examples like the "SYNOPSIS"
       above) uses "build_exporter", not "setup_exporter".  This means that
       the special arguments like "into" and "as" for "setup_exporter" are not
       accepted	here.  Instead,	you may	write something	like:

	 use Sub::Exporter
	   { into => 'Target::Package' },
	   -setup => {
	     -as     =>	'do_import',
	     exports =>	[ ... ],

       Finding a good reason for wanting to do this is left as an exercise for
       the reader.

       There are a whole mess of exporters on the CPAN.	 The features included
       in Sub::Exporter	set it apart from any existing Exporter.  Here's a
       summary of some other exporters and how they compare.

       o   Exporter and	co.

	   This	is the standard	Perl exporter.	Its interface is a little
	   clunky, but it's fast and ubiquitous.  It can do some things	that
	   Sub::Exporter can't:	 it can	export things other than routines, it
	   can import "everything in this group	except this symbol," and some
	   other more esoteric things.	These features seem to go nearly
	   entirely unused.

	   It always exports things exactly as they appear in the exporting
	   module; it can't rename or customize	routines.  Its groups ("tags")
	   can't be nested.

	   Exporter::Lite is a whole lot like Exporter,	but it does
	   significantly less: it supports exporting symbols, but not groups,
	   pattern matching, or	negation.

	   The fact that Sub::Exporter can't export symbols other than
	   subroutines is a good idea, not a missing feature.

	   For simple uses, setting up Sub::Exporter is	about as easy as
	   Exporter.  For complex uses,	Sub::Exporter makes hard things
	   possible, which would not be	possible with Exporter.

	   When	using a	module that uses Sub::Exporter,	users familiar with
	   Exporter will probably see no difference in the basics.  These two
	   lines do about the same thing in whether the	exporting module uses
	   Exporter or Sub::Exporter.

	     use Some::Module qw(foo bar baz);
	     use Some::Module qw(foo :bar baz);

	   The definition for exporting	in might look like this:

	     package Some::Module;
	     use base qw(Exporter);
	     our @EXPORT_OK   =	qw(foo bar baz quux);
	     our %EXPORT_TAGS =	(bar =>	[ qw(bar baz) ]);

	   Using Sub::Exporter,	it would look like this:

	     package Some::Module;
	     use Sub::Exporter -setup => {
	       exports => [ qw(foo bar baz quux) ],
	       groups  => { bar	=> [ qw(bar baz) ]}

	   Sub::Exporter respects inheritance, so that a package may export
	   inherited routines, and will	export the most	inherited version.
	   Exporting methods without currying away the invocant	is a bad idea,
	   but Sub::Exporter allows you	to do just that	-- and anyway, there
	   are other uses for this feature, like packages of exported
	   subroutines which use inheritance specifically to allow more
	   specialized,	but similar, packages.

	   Exporter::Easy provides a wrapper around the	standard Exporter.  It
	   makes it simpler to build groups, but doesn't provide any more
	   functionality.  Because it is a front-end to	Exporter, it will
	   store your exporter's configuration in global package variables.

       o   Attribute-Based Exporters

	   Some	exporters use attributes to mark variables to export.
	   Exporter::Simple supports exporting any kind	of symbol, and
	   supports groups.  Using a module like Exporter or Sub::Exporter,
	   it's	easy to	look at	one place and see what is exported, but	it's
	   impossible to look at a variable definition and see whether it is
	   exported by that alone.  Exporter::Simple makes this	trade in
	   reverse: each variable's declaration	includes its export
	   definition, but there is no one place to look to find a manifest of

	   More	importantly, Exporter::Simple does not add any new features to
	   those of Exporter.  In fact,	like Exporter::Easy, it	is just	a
	   front-end to	Exporter, so it	ends up	storing	its configuration in
	   global package variables.  (This means that there is	one place to
	   look	for your exporter's manifest, actually.	 You can inspect the
	   @EXPORT package variables, and other	related	package	variables, at

	   Perl6::Export isn't actually	attribute based, but looks similar.
	   Its syntax is borrowed from Perl 6, and implemented by a source
	   filter.  It is a prototype of an interface that is still being
	   designed.  It should	probably be avoided for	production work.  On
	   the other hand, Perl6::Export::Attrs	implements Perl	6-like
	   exporting, but translates it	into Perl 5 by providing attributes.

       o   Other Exporters

	   Exporter::Renaming wraps the	standard Exporter to allow it to
	   export symbols with changed names.

	   Class::Exporter performs a special kind of routine generation,
	   giving each importing package an instance of	your class, and	then
	   exporting the instance's methods as normal routines.
	   (Sub::Exporter, of course, can easily emulate this behavior,	as
	   shown above.)

	   Exporter::Tidy implements a form of renaming	(using its "_map"
	   argument) and of prefixing, and implements groups.  It also avoids
	   using package variables for its configuration.

       o   write a set of longer, more demonstrative examples

       o   solidify the	"custom	exporter" interface (see &default_exporter)

       o   add an "always" group

       Hans Dieter Pearcey provided helpful advice while I was writing
       Sub::Exporter.  Ian Langworth and Shawn Sorichetti asked	some good
       questions and helped me improve my documentation	quite a	bit.  Yuval
       Kogman helped me	find a bunch of	little problems.

       Thanks, guys!

       Please report any bugs or feature requests through the web interface at
       <>. I will be notified, and then you'll automatically
       be notified of progress on your bug as I	make changes.

       Ricardo Signes <>

       This software is	copyright (c) 2007 by Ricardo Signes.

       This is free software; you can redistribute it and/or modify it under
       the same	terms as the Perl 5 programming	language system	itself.

perl v5.24.1			  2013-10-18		      Sub::Exporter(3)


Want to link to this manual page? Use this URL:

home | help